This lecture reports on aspects of a larger research programme focused on studying mental mathematics in elementary and secondary mathematics classrooms. It specifically addresses an unplanned aspect that became salient through the work conducted in these classrooms. In this research programme, mental mathematics sessions are designed on a variety of mathematical topics (e.g., algebra, geometry, measurement, statistics, trigonometry, fractions), where students are given short amounts of time to solve tasks given orally and/or on the board, without the use of paper-and-pencil or any material aids. Whereas the central objectives centers on inquiring into the nature of the strategies students engage in to solve the tasks, more seem to be happening in these sessions. In particular, students' solutions and strategies to the task given in the mental mathematics context led to numerous questions, discussions, follow-up explorations, and so forth, by students, which in turn enabled the emergence of significant mathematical issues. This raised interest in investigating these (additional and unplanned) mathematical issues. This represents the core of this lecture, which focuses on the nature of the mathematics (in terms of content and of practices) that frequently unfolds in the mental mathematics sessions conducted. Using an illustrative extract from a mental mathematics session on analytical geometry in a Grade-10 classroom (15-16 years old), the analysis outlines how not only mathematical content is being worked on through these mental mathematics sessions, but also how mathematical practices are being enacted by students. This raises issues about the nature of the environment that these mental mathematics session plunge students into, one that could be tentatively, and boldly, aligned with Papert's concept of mathland. [For the complete proceedings, see ED629884.]